OK Adam, first of all nobody here is trying to 'con' anyone into buying anything. We have found that certain things work for certain issues and pass on the information. Simple as that.
Second, I will repeat my question for the third time. Did you compare your algae against the pictures available on this page to positively ID it? And if so, is it BBA(Black Beard/Brush Algae)?
If you did and found that it is, it will take a thorough cleaning and removal of the algae in order to rid yourself of it. Then the object is to re-plant and add additional plants and get them growing well. CO2 injection will provide the carbon necessary to push the plants which makes repeat infestations of many algae difficult, but is especially hard on BBA. If you notice, the infestation is probably mainly on areas of plants that are not thriving, where as the healthy, faster growing areas have less.
Once it's cleaned up and kept clean, and the plants are growing the algae will decline.
BTW, an SAE or two would also help. They eat this particular type of algae, but are not effective on the old, crusty stuff you've got. Once it's cleaned up they will eat most of the new stuff that may re-occur.
Now a piece of bad news. The CO2 kits that you see for sale are not very effective in a tank of more than 40 gals. or so. They simply can't push enough CO2 in to the water table for a tank with a volume of 90 gals.
IMO, your choices are either to go pressurized or to daisy chain four 2ltr. bottles of DIY CO2 mixture and with a good reactor it will aid the growth much more significantly than any kit(to my knowledge) on the market. Pressurized is the best option for you and by far the easiest way to feed a steady flow of gas into the tank.
One more suggestion. Get yourself enough substrate to create a minimum of 2" - 3" of material for your plants to grow in. 1", IMO is just not enough.
Len